While authorities have said many times that they have confidence in a timeline of Mollie Tibbetts' activities the day she vanished, few facts are known publicly.

Her boyfriend, Dalton Jack, has said that Tibbetts at some point sent him a picture using the messaging service Snapchat.

The value of Snapchat messages, or snaps, to a missing-person investigation is real but finite. "We work closely with law enforcement," the company explains online. But one of the service's best-known features — that the content of messages is deleted after recipients view them — limits what can be reconstructed later, Snapchat says.

"When you have people of a certain age, teenagers are using Facebook and Snapchat," said Iowa Division of Criminal Investigation Special Agent in Charge Mike Krapfl, speaking generally about investigations. "Some of the communications are lost and some of them are preserved by companies. You never know what you’re going to get."

In Tibbetts' case, Jack has said that he saw and opened her Snap at about 10 p.m. on July 18. It could have been sent earlier; authorities say the last confirmed sighting of her was at 7:30 p.m., while she was running in her hometown of Brooklyn, a small town of about 1,500 people between Des Moines and Iowa City. Jack was out of town, and Tibbetts had agreed to dog-sit at the home he reportedly shared with his brother and his brother's fiancee.

Jack has said he doesn't remember the content of the snap, other than that it appeared she was indoors in the image.

Tibbetts didn't turn up for work the next morning or respond to messages. A search began.

What is Snapchat?

Unlike other popular social media platforms or text apps, messages on Snapchat are designed to be temporary.

Snapchat lets app users send Snaps, typically consisting of photos or video, that disappear after they've been viewed. (The app sends notifications to senders if a recipient takes a screenshot, although there are ways to get around that.)

The messaging app is particularly popular among teens. A 2017 survey done by the Associated Press-NORC Center for Public Affairs Research found that nearly three-quarters of all teens use Snapchat and Instagram, another social app. That's more than the two-thirds of teens who say they use Facebook.

For some teens and young adults, Snapchat is their preferred way of communicating — even more so than text messages, which investigators can easily pull and examine.

"One of the main areas where Snapchat comes into play is with millennials and Gen Z," said Capt. Tony Leonard of the Georgia Tech Police Department. "More of their communication takes place through Snapchats and other apps than it does through texting or phone conversations like someone my age."

Investigators looking for a missing person will search for certain details in a photo posted on social media, including where it was taken and by whom, said Steve Martin, professor of criminal justice at Des Moines Area Community College's Ankeny campus.

For the most part, it's simple for law enforcement to get permission from companies like Snap Inc., which owns Snapchat, to provide a person's data. In an online law enforcement guide, Snap Inc. advises that it maintains a database of users' information that includes:

Snapchat username

Email address

Phone number

Snapchat user vanity name

Snapchat account creation date and IP address

Timestamp and IP address of account logins and logouts

The company also retains logs of previous snaps, stories and chats for 31 days. Logs contain metadata about these messages, such as the time stamp of a message and what account it was sent from.

Law enforcement can request the log from Snap inc. with proper court documents, such as a search warrant or subpoena.

In a transparency report by Snap Inc., between July 1 and Dec. 31, 2017, Snapchat said it received 5,094 requests for users' data and provided information for 88 percent of those requests in the United States.

The process to request data from companies like Snap Inc. is easy, said Ryan Getty, a criminal justice professor at California State University in Sacramento.

Officials will call a law enforcement liaison at the company and request that information about a person be preserved, while, at the same time, officials work to get proper documentation from the court system.

Companies will often send over the most recent data available and provide more extensive information later on physical media, Getty said.

"As long as there’s a real need immediately, they’ll do it while you’re on the phone," Getty said.

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Snapchat's limits

The vanishing nature of messages on Snapchat makes investigations into the data of a missing person more difficult and less reliable, in comparison with Facebook or Instagram, said Leonard, the Georgia Tech officer.

While Snapchat retains the metadata of the messages, it doesn't keep the actual content. If a detective wanted to view a story that was posted more than 24 hours ago on the app, that content is no longer available.

"It becomes a challenge for the investigators because not only is it (time) limited, (but) if the snap was already viewed, you may not be able to recover it all," Leonard said.

But even without specifics from Snapchat messages, cellphones themselves are useful to investigators because they are constantly in contact with cell towers and indicating their general location. However, in areas that are more rural, such as Brooklyn, there are fewer cell towers to provide more precise locations, Getty said.